Fuel control



Nov. 14, 1961 R. J. WENTE 3,008,515

FUEL CONTROL Filed May 28, 1957 2 Sheets-Sheet 1 CONTROL $E7'7//Y6`SPEED B /A/Er TEM/2 n 0MB oar @ewa/e5 L Taeg/NE TEMP,

Tram/5y Nov. 14, 1961 R. J. wl-:NTE 3,008,515

FUEL CONTROL Filed May 28, 1957 2 Sheets-Sheet 2 COLD DAY ITAA/MPO DAVCORRECTED Fo@ r/A/vo Hq SHAFT HORSEPQWE? FUEL FLOW L (U70/7F' VALVEOPENED IN VEN TOR.

United States Patent Office 3,008,515. Patented Nov. 14, 1961 3,008,515Y FUEL CONTROL Robert J. Wente, Indianapolis, Ind., assignor to GeneralMotors Corporation, Detroit, Mich., a corporation of Delaware Filed May28, 1957, Ser. No. 660,494 17 Claims. (Cl. 15S-36.4)

My invention relates to fuel controls for gasturbine engines,particularly aircraft engines.

The purpose of the invention is to provide improved control of the fuelat low power settings of the engine. The principal advantage of thecontrol according to the invention is that it provides a more accuratecontrol of engine power output -and holds the power substantiallyconstant at low power settings such as are used in the landing approachof an aircraft. Another advantage of -the invention is that it providesprotection against flameout of the engine at low power settings.

The invention preserves the advantages of the highly developed mainengine fuel control which is retained for controlling starting landacceleration of the engine, operation at the higher power settings, anddeceleration.

By way of background, it may be stated that the main fuel control of agas turbine is principally concerned with maintaining a desired turbinetemperature 4and preventing excessive turbine temperature. This may beaccomplished b y direct measurement of the turbine inlet temperature andmetering of fuel in response to the measurement. It may be doneindirectly by measuring such quantities as compressor inlet ram pressureor compressor outlet pressure, compressor inlet temperature, and

engine shaft speed, and deriving from these quantities, by a computingmechanism, the proper setting of a fuel metering valve to produce thedesired turbine temperature, My invention is preferably used incombination with a main fuel control which combines the control bydirect measurement of turbine temperature andthe indirect control fromother quantities referred to above. A main fuel control of this type,since it must handle large fuel fiows for 4full power engine operation,is lacking in precision of fuel metering at the lowl flows required forHight idle operation during a landing approach. Also, by the nature ofsuch controls, they vary the engine power output as ambient temperatureand press-ure and forward speed of the aircraft change, which isundesirable in -a landing approach. They may also, in response to acontrol malfunction, momentarily cut oif the fuel to the engine when itis operating Iat a low power setting, thus stopping combustion.

According to my invention, a second fuel'control is provided which isparticularly adapted to handle small fuel flows, such as occur at idlingoperation, and which responds to the controlling factors in such a waythat engine power output is held constant rather than turbinetemperature. This presents no hazard to the turbine, since thetemperature level at idling power levels is much below the maximum-allowable and the low power fuel control cannot supply a dangerousquantity of fuel.

The low power fuel control according to Athe invention meters fuel tothe engine in response to the values of ambient air pressure, apart fromlany ram effect, and the temperature of the air entering the engine.With fuel metering based on and compensated for these factors, theAengine power output can be held very closely constant at a low levelnotwithstanding variations in ambient temperature and aircraft speed.

'Phe control system according to the invention also preferably includesan engine speed-responsive valve which cuts otf the low power fuelcontrol lat speeds below the idling operational speed of the engine,such .as

are encountered only when the engine is being started,

so that starting may be' accomplished under control of the main jfuelcontrol, .which includes mechanisms to assure satisfactory starting.

The nature of the invention and the advantages thereof will be apparentto those skilled in the art from the succeeding detailed description ofthe preferred embodiment of the invention and the accompanying drawingsthereof.

FIGURE l is 'a schematic diagram of a fuel control system connected to aturboprop aircraft engine.

FIGURE 2 isa graphical illustration of the functioning and advantages ofthe control.

FIGURE 3 ispa graphical representation of the significance of the lowspeed cutoff.

Referring lto FIGURE 1, there is illustrated a gas turbine engine E, ofany suitable structure, which is coupled to a speed-governing propellerP through a shaft S and a reduction gear R. Fuel is supplied to theengine by a pump 10 driven from the engine through gears 11 and shaft'12. The normal path of fuel' from the pump to the engine is through aline 13, a main fuel metering valve 14, line 16, a resistance valve 17,line 18, a fuel shutoff valve r19, and line 20, which is connected tothe fuel nozzles (not shown) of the engine.

Since the main fuel control is well known, it is shown here onlydiagrammatically in the interest 0f conciseness and clarity. It isillustrated sufliciently |to show the relation therewith of the presentinvention. The main metering valve 14 is illustrated as comprising asleeve or cylinder 21 iand a valve spool 22 slidable in the cylinder. Asillustrated, Ithe spool 22 has closed the outlet line 16. The spool ismoved'upwardly to supply fuel to the engine by a controlmechanismindicated as la main fuel computer 23 connected through abellcrank 24 to the spool 22. 'Ihe main fuell computer responds to thecontrol setting, which is the setting of the pilots power control lever,to speed, which is the rotational speed of the engine, to inlettemperature, which is the temperature of the air entering the engine, tocompressor outlet pressure, which is the pressure of the air deliveredby Ithe compressor of the gas turbine, and to turbine temperature, whichpreferably is the temperature of the combustion gases entering theturbine. Ram yair pressure in the engine inlet may be, and frequentlyis, used for controlinstead of compressor outlet pressure. The computeropens the fuel valve 14 byan amount suicient to pass the desired orcomputed quantity of 'fuel to the engine. The metering head across valve14 is maintained constant at a suitable value, such as 30 lbs. per sq.in., by a bypass valve 24 which bypasses fuel from the pump outlet line13 into aline 26 connected to the pump inlet line 27. The bypass valvecomprises a throttling piston 28 biased downwardly, as shown, to openthe bypass by pressure in line 13 ahead of metering valve 14 and biasedupwardly to close the bypass by a spring 29 and by pressure in theoutlet of the metering valve communicated from line 16 through apressure connection 31. The actual structure of such controls is quitecomplex and, since it is understood by those yskil-led in the art, thereis no need to enterinto a discussion of the details of such a control.

The valve 19 is a standard element which is provided `to shut offcompletely the flow of fuel to the engine. It

is `always fully open when the engine is in operation. The resistancevalvel 17 serves to transfer control of fuel from the main meteringvalve 14 to the small flow control of the invention when the meteringvalve 1,4 is nearly closed. Valve vr17 is a valve of known type which aplug 33 is urged against a seat by a compression spring 34. Valve 17thus maintains la substantially constant pressure drop ofpreferablyabout 15 lbs. per sq. in. between lines 16 and 18 when theflow opens the valve. If the pressure difference lbetween lines 16v and18 is insufficient to overcome spring 34, the valve will close andpreventl flow A, from the main metering yvalve through line 18` to theengine. Valve :17 will close at small -ow rates because there is analternate path from line 16 to -the engine through the lower powerfuelcontrol of the invention.

This path comprises a -line 36 branching off the main metering valveoutlet line 16, an 'aneroid controlled throttling v-alve 37, a line 38,a speed controlled cutoff valve 39, and line 41 connected to line 18.There is also a bypass around the main metering valve from line 13through line 42, a makeup valve 43, and branch line 44 connected to line16. The makeup valve 43 -m-ay compn'se a spool 46-slidable in a cylinder47 and operative to throttle ilow between lines 42 and 44. Spool 46 ismoved by a rocker arm 48 pivoted at 49 and connected through a stem 51to a flexible diaphragm 52. Chamber 53 below diaphragm 52 is connectedto line 18, which is at the pressure P3 of fuel supplied to the engine.Chamber 54 above the diaphragm is connected to line 44, and thus is atthe pressure P2 in -l'ines 44, 1-6, and '36. The. difference between P2and P3 is the head which causes fuel to. ilow through throttling valve37 when cutoff valve 39 is open.

Rocker arm v48 is biased by a comparison spring 56 acting to bias makeupvalve 43 toward open position. The spring 56 is variably loaded byslidably mounted cam follower 57 actuated by a reciprocable cam 58. Theposition of cam 58 is determined byy temperaturefesponsive bellows 59connected to the cam and connected through line 61 to the inlettemperature-sensing bulb 62 in the inlet of the engine. The function ofthe makeup valve will be clarified later, but it may be pointed out atthis time that it passes fuel when the main metering valve 14 is closedand it controls the pressure drop through throttling valve 37.

Throttling valve 37 comprises a valve plunger 63 slidable in a wall 64between chambers 66 and 67 and including a contoured metering rvalveportion or needle 68 sooperating with a metering orifice I|59 betweenchamber 67 and chamber 71. The inlet line 36 to the valve is connectedto chamber 67 and the outlet line 38 is connected to chamber 71. Chamber71 is connected to chamber 66 through line 72 to balance the pressureson the ends of the plunger 63. Plun-ger 63 is actuated by a rocker arm73 pivoted at 74 in the valve body and actuated by an evacuated bellowsor aneroid 76'. Bellows 76 is mounted in a chamber 77 to which air atambient static atmospheric pressure (not ram pressure) is admittedthrough an opening 78. The position of valve plunger 63 and the amountof opening of the valve orifice is thus determined by ambient pressure.The relation between ambient pressure Iand the size of the opening isdetermined by the contour of needle 68,.

The cutoff valve 39 is provided as an automatic means to disable the lowpower fuel control during starting of the engine. It comprises a valvespool 79 reciprocable in a cylinder 81 under control of speed-responsivedevice 82 driven by shaft 112 through gears 83. The valve is shown inthe closed position. As the engine approaches the operating speed range,the flyball device -82 opens valve 39 so that it has no eect on the fuelsystem duringoperation of the engine, once it has been started.

'Ihe manner in which the low power fuel cont-rol operates may bedescribed by assuming that the engine is in normal operation at a cruisepower level and the power is reduced for landing. It is customary inlanding turboprop aircraft engines to reduce the shaft horsepower of theengines to a value slightly less than zero, which is Vreferred to as theHight idley operating condition of the engine, with the propellerlgoverning engine speed. The reason for operating the engine at anegative power output is that the landing is facilitated if the engineexerts a slight drag on the aircraft rather than propellingl it forward.The negative horsepower is not great. In an engine rated at 3,500 shaftH.P. maximum, the night idle output might be about minus 250 HP.Obviously,

the fuel requirement for this low output is much less than that for fullpower, roughly a third as much.

If the engine is in normal Hight, the main fuel computer will be set toa level toward the top of the power output range and the main fuelcontrol will be metering fuel to ymaintain a turbine temperature nearthe maximum. Fuel Iwill flow from pump 10 through t-he main meteringvalve 14 and resistance valve 17 to the engine. The excess dischargefrom the pump will be returned to the pump inlet through the bypassvalve 2,8 which holds the metering drop across valve 14 constant.

Now, if the power control is moved toward a lower power setting, t-hevalve 14 will be moved toward closed position and the engine poweroutput will decrease. At all times, in addition to the fuel flowingthrough resistance valve. 17 and line 118, some fuel will also beflowing through line 36, valve 37, line 38, val-'ve 39, and line 41.This does not aifect the amount of fuel supplied to the engine in normaloperation, since the fuel has already been metered by valve 14. However,as the fuel rate decreases, a ow rate will be reached at which all thefuel can iiow through valve 37` at the metering head established by theresistance valve v17 and the resistance valve Iwill be closed. When themain control is set to ight idle, the main metering valve will closeentirely. When the main valve is closed, fuel flows to the enginethrough line 42, makeup valve 43, lines y44, 16, and 36, and throttlingvalve 37. Valve 46 is referred to as a makeup valve because it suppliesor makes up a suflicient quantity of fuel to operate the engine atflight idle i-f the main metering valve does not supply it. Valve 43 cando this if the main metering valve is nearly but not quite closed, andthe main valve need not be entirely closed at ight idle, so long as itis so nearly closed that valvea17 closes.

As previously pointed out, spring 56 tends to open makeup valve 43,which is a balanced valve. The difference between P2 and P3, which isthe metering head across throttling valve 37, tends to close 'valve 43.If the main metering valve 14 closes and valve 43 is closed, there is noflow through valve 317 or through valve 17, and thus P3 rises to equalP2. This allows spring 56 to open valve 43 and the resulting pressuredrop across valve 37 acts to close valve 43 in opposition to spring 5'6.Thus,'for any given loading of spring 516, valve 43 will automaticallylopen suiciently to maintain a predetermined pressure drop between P2 andP3 across the throttling valve 37. If sufficient fuel is passed by themetering valve to maintain this pressure differential, the makeup valvewill close. The value of the pressure dijerential between P2 and P3 istherefore controlled by the loading of spring 516, which is varied byengine inlet temperature, sensed by bulb 62, and acting throughtemperature bellows 5.9, cam '58 and follower 57. Thus, the pressuredrop across the metering orifice y69 is controlled by inlet temperature.

This is one of the two factors which determines ow through valve 37, theother being the opening of the orilice by needle 68. As previouslyexplained, this opening is a predetermined function of ambientatmospheric pressure as a result of the operation of the plunger v63 bythe aneroid 76 and the predetermined contour of the needle. The flow tothe engine is, therefore, determined by ambient pressure and inlettempera-ture. These two factors are particularly suited to maintainconstant engine power output notwithstanding changes in forward speed ofthe aircraft, ambient pressure, and ambient temperature.

Therefore, when the engine control is set to flight idle during alanding approach, the low power fuel control takes over 'from the mainfuel control and holds the engine quite accurately at the desired ightidle power output.

It will be understood, of course, that the contours of cam 58 and valveneedle 68 will be determined for any given installation to t thecharacteristics of the particular engine, which will vary from one modelof engine to another. It is possible to determine by tests the desiredfuel ow to maintain the desired flight idle power output as a functionof ambient pressure and inlet temperature and to shape the cam 5-8 so`that it maintains power substantially constant by varying the llow asnecessary to compensate for changes in inlet temperature. Likewise,needle 68 is contoured so that the effective area of the orifice variesto regulate the flow so that i-t is varied in such a way as to maintainthefpower constant notwithstanding changes in ambient pressure.

The curves of FIGURE 2 illustrate the advantages of my cont-rol asapplied to an example of a turboprop engine. FIGURE 2 illustrates thevariation of engine shaft horsepower with 'air speed and inlet airtemperature, for a low power setting of the main engine control. Airspeed, which is the forward speed increasing the compressor inletpressure, tends to increase power output. With a control which tends tomaintain constant turbine temperature, the power increases as the airspeed increases. trol. However, the low power fuel control, whichresponds to Static ambient pressure, does not change power with changesin forward speed. AAlso, the powerlevel is greater as ambienttemperature is lower when turbine temperature is held constant, as it isby the main cont-rol. 'I'.hus, the curve cold day illustrates therelation of power output to air speed on avery cold day.- The curveidentied as standard day is a similar curve at standard atmospherictemperature, and the curve identified as hot day shows the power curveon a very hot day. As will be apparent, regardless of the temperature,the power level curve goes up with increase in air speed. Also, theoverall ordinates of the curve are depressed as ambient temperatureincreases.

The desired result is to eliminate these variations. By suitablycorrecting the fuel iiow for engine inlet temperature, indicated by thedotted lines identified'as corrected for T1, the power level can be heldsubstantially constant regardless of variations in ambient temperature,lbut the disturbing effect of aircraft speed remains. If compensationfor the pressure of the air is made by metering fuel in accordance withstatic ambient pressure (rather than ram pressure or compressordischarge pressure as in the typical main fuel control) so that the rameffect of forward speed of the aircraft does not increase the fuel flow,the power may be held substantially constant as indicated by the linescorrected lfor T1 and PA. The power is held very closely to the desiredslightly negative flight idle value and is not signiicantly changed bychanges in temperature of the air as the plane approaches the ground ordecreases in speed as the landing approach is made.

`Considering FIGURE 2 in terms of the system shown in FIGURE l, thefunction of -the cam 58 is to make the correction for. inlet temperatureand the function of needle 68 is to make the correction for airpressure.

Since a set of characteristic curves of power output as a function ofambient pressure and inlet temperature can be made for any engine, theproper contours of the cam 58 andthe needle 68 can be determined.

FIGURE 3 illustrates the reason for the starting cutoff valve 39. Thisgure shows the fuel flow required in starting the engine as a functionof engine speed. At the beginning of the introduction of fuel duringstarting, the fuel flow is below the corrected minimum flow which wouldbe established by the low power fuel control. As the starting cycleprogresses, the fuel rises above the corrected minimum ow line.Somewhere in this range, as indicated by the vertical dotted line, thecutoifvalve 39 is opened, activating the small flow fuel control. Thisis at a speed below the flight idle operating speed of the engine. Inthis connection, the flight idle operating This is characteristic of themain fuel conspeed maybe the same as full power operating speed and, inmost cases, will be near the full operating speed;

The advantages of the small ow control may be reviewed brieflly. Theymake it possible for the pilot yto be. assured of constant power outputfrom each engine and equal power outputs frompall engines during thelanding approach at the value established as most desirable. The lowpower fuel control also maintains fuel flow to the engine at a valuesufcient to maintain combustion even if the main metering valve closes.If the .power lever is set at a low level, the metering valve will benearly closed. Under these conditions, it is possible for a controlmalfunction to cause the main metering valve to close momentarily. If itwere not for the. low power fuel control which maintains the flow, thiswould temporarily interrupt flow to the endine. This would cause theflame to go out in theengine and resumption of fuel flow would notre-establish combustion. A power failure for this reason during alanding might result in a casualty. It will be apparent that anysuitable valve mechanism such kas a manually operated valve could beused to cut out the low power fuel control for starting purposes. Itwill also be`=apparent that other fuel metering arrangements responsiveto inlet temperature and ambient pressure could be substituted for thepreferred arrangement illustrated and described. In particular, thefunctions of inlet temperature and ambient pressure could Ibe reversedso 'that inlet temperature controls the throttling valve opening andambient pressure controls the metering head across this valve. Thearrangement shown is preferred, however.

The description of the preferred embodiment of the invention for thepurpose of explaining the principles thereof is not to be construed aslimiting or restricting the invention, since many modifications may bemade by the exercise of skill in theart within the scope of theinvention. Y

I claim: A v

l. A fuel system fora gas turbine engine comprising, in combination,means for supplying lfuel, a main fuel control adapted-to meter fuel forvarying engine power requirements up to maximum power requirement, and'a low power fuel control responsive to ambient atmospheric pressure andenginel inlet temperature adapted to meter fuel so as to maintain powersubstantiallyv invariant with ambient atmospheric conditions at a lowpower level, the main and low power fuel controls being connected inparallel between the fuel supplying means and the engine.

2. A fuel system for a gas turbine engine comprising, in'combination,means for supplying fuel, a main fuel control adapted to meter fuel forvarying engine power requirements up to maximum power requirement, a lowpower fuel control adapted to meter fuel variably so as to maintainpower substantially invariant with ambient atmospheric conditions at alow power level, the main and low power fuel controls being-connected inparallel between the fuel'supplying -means and the engine, meansoperative to disable the main fuel control below a predetermined fuelflow rate, and means responsive to a condition of engine operationoperative to disable the'low power fuel control for starting the engine.

3. A fuel system for a gas turbine engine comprising, in combination,means for supplying fuel, a main fuel control adapted to `meter fuel forvarying engine power requirements up to maximum power requirement, a lowpower fuel control adapted to meter fuel so as to maintain powersubstantially invariant with lambient atmospheric conditionsat a lowpower level, the main and low power fuel controls being connected inparallel between the fuel supplying means and the engine, meansresponsive to lfuel flow rate through the low power fuel controloperative to disable the main fuel control below `a predetermined enginepower level, and means responsiveto engine speed operative to disablethe low power fuel control for starting the engine.

4. A fuel system for a gas turbine engine comprising, in combination,means Ifor supplying fuel; a main fuel control connected -between thesupplying means and the engine adapted to meter fuel up to 4the maximumengine demand; and a low power fuel control capable of meteringsuliicient fuel for engine operation near zero power output connectedbetween the supplying means and the engine, the low power fuel controlcomprising fuel metering means responsive to engine inlet temperatureand ambient atmospheric pressure operative to maintain engine poweroutput substantially invariant with changes in said temperature andpressure at a predetermined idling power output level.

5. A fuel system for a gas turbine engine comprising, in combination,means for supplying fuel; a main fuel control connected between thesupplying means and the engine adapted to meter fuel up to the maximumengine demand; and a low power fuel control capable of meteringsufficient fuel for engine operation near zero power output connectedbetween the supplying means and the engine, the low power fuel controlcomprising, in series, a makeup valve and a throttling valve, meansresponsive to engine inlet temperature, and means responsive to ambientatmospheric pressure, each of said means controlling one of said valves.

6. A fuel system for a gas turbine engine comprising, in combination,means for supplying fuel; a main fuel controlconnected between thesupplying means and the engine adapted to meter fuel up to the maximumengine demand; and a low power fuel control capable of meteringsufficient .fuel for engine operation near zero power output connectedbetween the supplying means and the engine, the low power fuel controlcomprising, in series, a makeup valve and a throttling valve, meansresponsive to engine inlet temperature and the pressure drop across thethrottling valve connected to the makeup valve so as to open the makeupvalve to maintain a pressure drop varied lby inlet temperature acrossthe throttling valve, and means responsive to ambient atmosphericpressure connected to the throttling valve to open the valve as arfunction of atmospheric pressure.

` 7. A fuel system for a gas turbine engine comprising, in combination,means` for supplying fuel; a main fuel control connected between thesupplying means and the engine adapted to meter fuel up tothe maximumengine y`demand; and a low power `fuel control capable of meteringsufficient fuel for engine operation near zero power output connectedbetween the supplying means and the engine, the llow power -fuel controlcomprising, in series, a makeup valve, a throttling valve, and a cutoffvalve, means responsive to engine inlet temperature and the pressuredrop across the throttling valve connected to the makeup valve so as toopen the makeup valve to maintain a pressure drop varied by inlettemperature across the throttling valve, means responsive to ambientatmospheric pressure connected to the throttling valve to open the valveas a function of atmospheric pressure, and means responsive to enginespeed connected to the cutoff valve adapted to close the cutoff valve ata speed below normal engine operating speed.

8. A fuel system ,for a gas turbine engine comprising, lin combination,means for supplying fuel; a main fuel control connected between thesupplying means and the engine adapted to meter fuel up to the maximumengine demand; and a low power fuel control capable of meteringsufficient fuel for engine operation near zero power output connectedbetween the supplying means and the engine, the low power fuel controlcomprising, in series, a makeup valve, a throttling valve, and a cutolfvalve, means responsive to engine inlet temperature and the pressuredrop across the throttling valve connected to the makeup valveso as toopen the makeup valve to maintain a pressure drop varied by inlettemperature across the throttling valve, means responsive to ambientatmospheric pressure connected to the throttling valve to open the valveas a function of atmospheric pressure, and means responsive to enginespeed connected to the cutoff valve adapted to close the cutolf valve ata speed below normal engine operating speed; and a resistance Ivalveconnected between the main fuel control and the engine.

9. A fuel `system for a gas turbine engine comprising, in combination,means for supplying fuel; a main fuel control connected between thesupplying means and the engine adapted to meter fuel up to the maximumengine demand; and a low power fuel control capable of metering suicientfuel for engine operation near zero power output connected between thesupplying means and the engine, the low power fuel control comprising,in series, a throttling valve and a cutoff valve, means responsive toambient atmospheric pressure and temperature connected to the throttlingvalve to control the ow through the valve, and means responsive toengine speed connected to the cutoff valve adapted to close the cutoffvalve at a speed below normal engine operating speed.

10. A gas turbine Vfuel control comprising, in combina,- tion, main fuelmetering valve means adapted to supply fuel for varying engineoperations from near idle to full power, including means for variablycontrolling the power output of the engine, and low power fuelthrottling means adapted to supply fuel to the engine at a rate causingsubstantially constant power output of the engine at an idling level,means responsive to closing of the main fuel metering valve meansconnected to the low power fuel throttling means to activate the lowpower fuel throttling means, means sensitive to ambient atmosphericpressure and temperature, and means connecting the sensitive means tothey low power fuel throttling means for control thereof; the sensitivemeans, the connecting means, and the low power fuel throttling meansbeing so constructed as to vary fuel flow in response to the ambientatmospheric pressure and temperature so as to maintain engine poweroutput at a substantially constant idling level, notwithstandingvariation of the ambient pressure and temperature.

11. A fuel control as recited 4in claim l0 including means responsive toengine speed connected to the low power fuel throttling means adapted toshut off flow through the low power fuel throttling means when enginespeed is below a predetermined value less than the normal operatingspeed of the engine.

12. A fuel control as recited in claim 10 in which the means responsiveto closing of the main fuel metering valve means is a resistance valveconnected in series with the main fuel metering valve means.

13. A gas turbine fuel system comprising, in combination, means forsupplying fuel, a main fuel metering valve supplied thereby, land meansconnecting the metering valve to the engine adapted to provide apredetermined pressure drop in response to `flow through the saidconnecting means, the main metering valve being dimensioned to supplythe maximum fuel requirement of the engine, a iirst throttling valveconnected in parallel with the main metering valve, a second throttlingvalve connected between the main metering valve and the engine, meansresponsive to the pressure drop across one throttling valve connected-to the other throttling valve and actuating the said other throttlingvalve to control the pressure drop -across the said one throttlingvalve, means responsive to ambient atmospheric pressure biasing onethrottling valve, and means responsive to ambient atmospherictemperature biasing the other throttling valve, the throttling valvesbeing so contoured that the throttling valves maintaina' fuel owcompensated -for ambient pressure and temperature so as to maintainengine power substantially constant at an idling level.

14. A gas turbine fuel system as recited in claim 13 in which the meansresponsive to ambient atmospheric tern- Perature is connected to theiISt throttling valve and means responsive to ambient atmosphericpressure is connected to the second throttling Valve.

15. A fuel system for a gas turbine engine comprising, in combination,means for supplying fuel; a main fuel control adapted to meter fuel upto the maximum engine demand including a main metering valve connectedbetween the supplying means and the engine and a resistance valveconnecting the main metering valve to the engine, the resistance valveopening in response to a predetermined pressure drop thereacross; and alow power fuel control capable of metering suficient fuel for engineoperation near zero power output, the low power fuel control comprisinga lirst throttling valve connected in parallel with the main meteringvalve and a second throttling valve connected in parallel with theresistance valve, control means responsive to the ambient atmosphericvalues of pressure and temperature, respectively, one said control meansbeing connected to each said throttling valve, means responsive to thepressure drop across the parallel resistance valve and second throttlingvalve connected to the iirst throttling valve biasing the iirstthrottling valve in an opening direction in response to decrease in thesaid pressure drop, the said control means and the said throttlingvalves being so constructed as to regulate the fuel flow through thethrottling valves to compensate the effects of ambient pressure andtemperature on engine power to maintain a substantially constant lowengine power output.

16. A fuel system for a gas turbine engine coupled to a variable loaddevice including governing means adapted to vary the load imposed on theengine so as to maintain engine speed at a predetermined value, the fuelsystem comprising, in combination, means for supplying fuel; a main fuelcontrol adapted to meter fuel up to the maximum engine demand includinga main metering valve connected between the supplying means and theengine and a resistance valve connecting the main metering valve to theengine, the resistance valve opening in response -to a predeterminedpressure drop thereacross; and a low power fuel control capable ofmetering suiiicient fuel for engine operation near zero power output,the low power fuel control comprising a iirst throttling valve connectedin parallel with the main metering valve and a second throttling valveconnected in parallel with the resistance valve, control meansresponsive to the ambient atmospheric values of pressure andtemperature, respectively, one said control means being connected toeach said throttling valve, means responsive to the pressure drop acrossthe parallel resistance valve and second throttling valve connected tothe rst throttling valve biasing the first throttling valve in anopening direction in response to decrease in the said pressure drop, thesaid control means and the said throttling valves being so constructedas to regulate fuel ow through the throttling valves to compensate theeffects of ambient pressure and temperature on engine power to maintaina substantially constant low engine power output.

17. A fuel system as recited in claim 16 including valve meansresponsive to engine speed connected in series with the low power Ifuelcontrol and operative to shut off fuel flow through the low power fuelwhen engine speed falls substantially below the said predeterminedspeed.

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